Sorry to hear of your woes, benw..
For what it's worth, I have the FSX "Deluxe edition" and I installed the SDK from the DVD 1.
Gmax is not important but it was installed I guess from the SDK within that first DVD in the deluxe edition.
Not sure about other FSX editions as I only have the one I mentioned.
For me, it all worked flawlessly after the install of the SDK.
I had already installed Sp1 and 2 many moons before I even had my Rift.
Apart from the SP1 and 2 I have no other updates or whatever installed.
I was advised to contact Dan at support (email address is on the Shop page) about my difficulties and he solved the problem with his suggestion about the SDK on DVD1.
Maybe an idea for you to get in touch with him?
Anyway, all the very best.